The present invention relates to liquid level sensors and leak detectors for liquids held in a tank or container, such as a fuel tank.
The conventional device presently used to indicate the vehicle fuel tank level is the mechanical (float actuated) fuel gauge. Since it has mechanically moving parts, its reliability is relatively low. Moreover, the presence of mechanically moving parts and electrical connections to these parts can present a risk of unintentional fuel ignition, i.e., a fire hazard.
An evanescent wave absorption technique for measuring the length of a fiber immersed in a liquid is described in the issued and commonly assigned patent, number 5,077,482, issued Dec. 31, 1991, entitled "Fiber Optic Fuel and Liquid Gauge," by V. Vali et al., now U.S. Pat. No. 5,077,482, the entire contents of which are incorporated herein by this reference. When the cladding thickness of an optical fiber is only a few wavelengths of light, as in an eccentric core fiber, the evanescent wave extends outside the fiber cladding. If such a fiber is covered by or immersed in a liquid with index of refraction larger than that of the fiber cladding, some of the light leaks out of the fiber core. The effect can be used to determine the height of the liquid level in a container, as described in the referenced co-pending application.
The fiber evanescent wave fuel gauge (liquid level sensor) described in the referenced pending patent application works only when the index of refraction of the fiber cladding is less than the index of refraction of the fuel.
The physics of a fuel gauge as described in the pending application is shown in FIG. 1. Here, the fiber comprises a core 20 and cladding 22. The core is made of a material with index of refraction n.sub.1, and the cladding of thickness d is made of a material with index of refraction n.sub.2. The fiber is immersed in a liquid with index of refraction n.sub.3. The light energy density profile in the fiber is indicated by line 24, with the darkened area under the line 24 indicating the light loss from the fiber due to the evanescent wave loss. The light intensity E in the cladding 22, the evanescent wave, decreases exponentially with increasing r, the distance from the core center. EQU E=E.sub.0 e.sup.-.alpha.r ( 1)
where E.sub.0 is the core intensity (i.e., the intensity of light within the fiber core) and .alpha. is determined by the core and cladding indexes n.sub.1 and n.sub.2 : ##EQU1## where .DELTA.n=n.sub.1 -n.sub.2, and n=(1/2)(n.sub.1 +n.sub.2)
Numerically, for .lambda.=1 .mu.m, the index of refraction of the fiber core n=1.5, and .DELTA.n=10.sup.-3 (the nominal values for an exemplary fiber), EQU (1/.alpha.)=3 .mu.m (4)
If the cladding thickness d is made small enough such that part of the wave extends outside the fiber and if the index of refraction of the liquid n.sub.3 is larger than the cladding index n.sub.2, some of the light is lost from the fiber. In case of n.sub.3 &lt;n.sub.2 (for example, when the fiber in air) no light is coupled out. Therefore, the amount of light lost is an exponential function of the length of the fiber in the liquid and the quantity of fuel can be gauged.
Single mode fibers, however, cannot be made small enough (.about.15 .mu.m in diameter) to satisfy the condition that d be less than 5 .mu.m. For this reason an eccentric core fiber is used, as depicted in FIG. 2. The fiber 30 comprises a core 32 surrounded by a cladding 34. The normal fiber diameter (.about.120 .mu.m) is retained for handling and mechanical strength purposes; however, the core 30 is placed close to the fiber surface such that d&lt;5 .mu.m.
In an exemplary system as described in the referenced pending application and calibrated for d.apprxeq.3 .mu.m, about 75% of the light leaked out over a fiber length of six inches. It has been found that the light leak rate was very sensitive to the variations of d, the cladding thickness.
The condition n.sub.3 &gt;n.sub.2 can be satisfied for diesel fuel and oils (where the index of refraction n.sub.3 of the fuel and oils=1.45, and the index n.sub.2 =1.43 for quartz cladding).
When gasoline is the liquid the condition n.sub.3 &gt;n.sub.2 is not satisfied and no light leaks out of the fiber. The present invention overcome this difficulty.
It is an object of this invention to provide a fiber optic fuel gauge with no electrical parts in the fuel tank, to reduce the probability of unintentional ignition of the fuel.